Heretofore there have been efforts to utilize polycrystalline semiconductor films for several device purposes. Usually the small grain structure and lack of columnar grains has made the feasible device structures unsatisfactory for most purposes. The problem of the growth of the polycrystalline semiconductor material has been that it has precipitated directly from a liquid phase or from a solid phase and the consequence of such a growth is usually a grain structure that is comparable to thickness of the film. Further, the prior art procedures have provided growth of materials from gases by incorporating the semiconductor therefrom into a metallic liquid and the growth proceeds by precipitation of the semiconductor itself, leaving behind the metallic liquid.
In the prior art, U.S. Pat. No. 3,346,414 issued Oct. 10, 1967 for a vapor-liquid-solid crystal growth technique is of background interest. It discloses a process for the controlled growth of a crystalline body comprising a first material at a given site. The process comprises providing a second material comprising an agent at the said site, a contacting the said second material with a vapor comprising the said first material. The said agent is such that it is capable of forming a liquid solution comprising the said agent and the said first material. The said second material is maintained at a temperature above the initial freezing temperature of the said solution. The process includes continuing the said contacting of the vapor for a time sufficient to supersaturate the said solution with respect to the said first material thereby initiating crystallization of the said first material at the said site.
The teachings of the noted U.S. Pat. No. 3,346,414 involve the use of a liquid metallic material as an agent to promote the growth from a vapor phase of a semiconductor material onto a substrate. As the semiconductor material is grown on the substrate, the liquid remains over the material which has grown and remains in contact with the vapor. The liquid metallic material is essentially not consumed in the growth process. As a result, all of the material which is grown according to this prior art must have been in intimate contact with the metallic liquid growth agent. As impurity comprising the liquid metallic growth agent will remain in the material which has been grown, to a concentration which is as high as the solubility of the liquid material growth agent in the material being grown at the temperature of growth. This impurity in the grown material is deleterious to the operation of certain classes of electronic devices formed on the grown material.